This invention relates generally to protective devices for use in rotating machinery, and more particularly, to torque-limiting devices. In many applications of rotating machinery, it is necessary or highly desirable to limit the mechanical torque transmitting through a mechanical linkage, usually from an electric motor. Some rotating machinery, such as cable winches, are particularly susceptible to shock loading, when a cable suddenly takes up a load, or becomes snagged. In other cases, shock loading effects can result from the inertia of the rotating machinery.
One practical application in which slip clutches are subject to shock loading is that of an electric winch used in aircraft to retract a cable referred to as a static line. A static line is used in the automatic deployment of a parachute. When the line is winched back into the aircraft, possible snagging of the line on parts of the aircraft can jeopardize the safety of the crew unless there is some automatic safety device to minimize the effect of a snagged line. An electrical disconnect based on a predetermined motor current threshold does not act fast enough to be effective. The inertia of the motor and associated rotating machinery can increase the pull on the line above a safety level before the motor current reaches its cutoff limit. A mechanical disconnect device that sheds mechanical load upon reaching a torque limit may not be satisfactory either, since the line may unreel further when the load is disconnected.
There are several forms of fast-acting slip clutches that function to "clip" or limit the transmitted torque without releasing the load. When the excessive torque is no longer required, the clutch resets itself and transmits the normal rated load. Slip clutches of the prior art have a common failing in that they can tolerate slipping for only a short period of time, in some cases only a few seconds, before becoming overheated and failing.
Some slip clutches are provided with audible alarms that are actuated whenever there is clutch slippage. Examples may be found in U.S. Pat. No. 3,786,776 to Buthe et al., U.S. Pat. No. 3,756,042 to Heth et al., and U.S. Pat. No. 2,753,703 to McIntyre. Although an audible warning may be useful in some situations, it has the disadvantage of requiring operator intervention before protective action may be taken. Moreover, the alarm may not distinguish between a potentially harmful situation and one in which some degree of slippage would be desirable.
Some clutch devices measure relative clutch slippage by electrical and other means. For example, German Pat. No. 1,053,794 discloses an electrical circuit for detecting relative rotation of two parts of a clutch, and British Pat. Appln. No. 2,083,223 discloses the use of a piezoelectric device for indicating relative rotation of two parts of a clutch. Japanese Pat. Appln. No. 55-103125 discloses a variable-speed machine in which the speed of a drive shaft is monitored to determine whether there is clutch slippage. Motor current and oil pressure are also monitored in this machine, and an alarm is actuated if any abnormality is detected. Nothing in the prior art, however, addresses the specific problem inherent in all torque limiting devices, except to the extent of providing an alarm.
It will be apparent from the foregoing that there still exists a need for a slip-clutch device that not only effectively limits the transmitted torque, but also provides protection for the clutch itself. The present invention fulfills this need.